In an increasing number of various electronic devices such as portable phone and electronic cameras or car navigation systems, light-transmitting touch panels are mounted on upper surfaces of liquid crystal displays, and various functions of the electronic devices are switched with the touch panels. The number of types of electronic devices having touch panels mounted thereon has further increased, and the touch panels have been required to have resistance to environment.
FIG. 4 is a sectional view of conventional touch panel 30 described in Japanese Patent Laid-Open Publication No. 2008-070938. Touch panel 30 includes upper wiring plate 1, lower wiring plate 2, and adhesive layer 3. Upper wiring plate 1 includes upper substrate 11, upper conductive layer 12, upper electrode 13, and insulating layer 14. Lower wiring plate 2 includes lower substrate 21, lower conductive layer 22, lower electrode 23, insulating layer 24, and dot spacers 25. Light-transmitting upper conductive layer 12 having conductive property and a rectangular shape is formed on the lower surface of light-transmitting upper substrate 11. Light-transmitting lower conductive layers 22 having conducting property and a rectangular shape is formed on the upper surface of light-transmitting lower substrate 21. One pair of upper electrodes 13 are arranged at the left and right ends of upper conductive layer 12. One pair of lower electrodes 23 are arranged at forward and backward ends of lower conductive layer 22. End parts of upper electrode 13 and lower electrode 23 extend to respective front ends of peripheries of upper substrate 11 and lower substrate 21. A section of lower electrode 23 extending from a front end side of lower substrate 21 to a rear end side thereof appears below upper electrode 13 on the right. Insulating layer 14 and insulating layer 24 are made of acrylic resin. Insulating layer 14 covers upper electrode 13, while insulating layer 24 covers lower electrode 23. Dot spacers 25 are disposed on the upper surface of lower conductive layer 22. An adhesive agent constituting adhesive layer 3 is applied onto the peripheries of upper substrate 11 and lower substrate 21 to bond substrates 11 to substrate 12 while conductive layer 12 faces lower conductive layer 22 with a predetermined gap.
Materials of upper electrode 13 and lower electrode 23 will be detailed below.
Upper electrode 13 and lower electrode 23 are made of resin having silver powder and carbon powder dispersed therein. The resin is made of polyester containing epoxy resin. The content of silver powder ranges from 10 wt. % to 30 wt. %, while the content of carbon powder ranges 1 wt. % to 5 wt. %. In this case, the carbon powder has a particle diameter ranging from 30 nm to 40 nm and a Brunauer Emmett Teller (BET) specific surface area not smaller than 700 m2/g.
Upper electrode 13 and lower electrode 23 are made of the above materials to connect fine carbon powder particles into chain structures and to disperse the carbon powder particles between silver powder particles. For this reason, upper electrode 13 can be electrically connected with upper conductive layer 12 stably, and lower electrode 23 can be electrically connected with lower conductive layer 22 stably. Even if an operating environment of touch panel 30 changes, for example even if touch panel 30 is used in, for example, a high-temperature, high-humidity atmosphere, these materials stably maintain the electric connections between upper electrode 13 and upper conductive layer 12 and between lower electrode 23 and lower conductive layer 22.
Since the materials of upper electrode 13 and lower electrode 23 of conventional touch panel 30 are formed by dispersing silver powder and carbon powder in resin, resistances of upper electrode 13 and lower electrode 23 are larger to cause voltage to drop more than upper electrode 13 and lower electrode 23 made mainly of silver.
Touch panel 30 is arranged above a liquid-crystal display to be installed into an electronic device. Upper electrode 13 and lower electrode 23 are electrically connected to a control circuit, such as a microcomputer, of the electronic device.
Upon an operator having a finger or a pen depress the upper surface of upper substrate 11 according to a display of the liquid-crystal display behind touch panel 30, upper substrate 11 causes upper conductive layer 12 to contact lower conductive layer 22 at the depressed position. While applying a voltage from sequentially to upper electrode 13 and lower electrode 23, the control circuit detects the depressed position based on a voltage ratio of these electrodes to switch various functions of the device.